The present invention relates to impedance matching circuits for switched alternating current (AC) power supplies. In particular, this invention relates to an impedance matching circuit having an inductive input impedance for matching switched AC power supplies to electronic lighting elements such as, for example, LED optical assemblies of traffic light systems.
Traffic light systems using electronic lighting elements such as light emitting diodes (LEDs) provide advantages of lower costs, energy savings, better overall brightness and reliability compared with electrical lighting elements such as incandescent lamps or halogen bulbs. The LEDs used in such traffic light systems are typically arrayed to provide light signals at desired colors. Each array of LEDs is commonly referred to as an optical assembly.
Electrical AC power to the different lighting elements or optical assemblies of a set of traffic lights is typically provided with an alternating current (AC) power source. The AC power is switched to each of the different lighting elements or optical assemblies by a controller. Such switching enables the different traffic light signals to be provided as desired. The AC power is then rectified with a transformer to provide direct current (DC) power.
The electrical power required by electronic lighting elements is typically lower than that for electrical lighting elements. Consequently, transformers for electronic lighting elements require more primary windings to step down the AC power to desired DC levels. Furthermore, as electronic components are generally more temperature sensitive than electrical components, such transformers should preferably be efficient to minimize heat generation.
For an efficient transformer, less heat is dissipated through its coils compared with less efficient or lossy transformers. Hence, unused AC power during the return power is typically reflected back to the AC power source. However for a triac switched AC power source during periods of non-conduction, the reflected AC power is not absorbed or sunk but is reflected back. This is because of the high impedance of the triac during such non-conduction periods. The reflection of the AC power by the triac switched AC power source at zero-crossings of an AC power cycle is a known problem that causes distortion of the AC power cycle. A power cycle of a triac switched AC power source is illustrated in the waveform of FIG. 1 with two zero-crossing distortions 8a,8b indicated.
Distortion of the AC power cycle results in a non-symmetrical AC waveform that has a net DC component. Such a net DC component is known to cause undesired heat dissipation in AC transformers and damage components such as, for example, fuses of the optical assemblies. Therefore, a need clearly exists for traffic light systems having electronic lighting elements to operate reliably with triac switched AC power supplies.
The present invention seeks to provide an impedance matching circuit, an optical assembly having the impedance matching circuit and a traffic light system that comprises at least one of the optical assembly.
Accordingly, in one aspect, the present invention provides an impedance matching circuit for matching a switched alternating-current (AC) power supply to an optical assembly, the impedance matching circuit comprising:
at least one matching transformer electrically coupled to the switched AC power supply;
a first resistor chain comprising at least one resistor connected in series and having an input and an output, the input being common with an electrical node of the matching transformer, the output being common with a supply node of the optical assembly; and
a second resistor chain comprising at least one resistor connected in series and having an input and an output, the input being common with another electrical node of the matching transformer, the output being common with another supply node of the optical assembly.
In another aspect, the present invention provides an optical assembly for traffic light signaling, the optical assembly comprising:
a plurality of lighting elements;
an input transformer having primary windings and secondary windings, the secondary windings being electrically coupled to the plurality of lighting elements; and
an impedance matching circuit electrically coupled to the input transformer, the impedance matching circuit comprising:
at least one matching transformer;
a first resistor chain comprising at least one resistor connected in series and having an input and an output, the input being common with an electrical node of the matching transformer, the output being common with a supply node of the primary windings; and
a second resistor chain comprising at least one resistor connected in series and having an input and an output, the input being common with another electrical node of the matching transformer, the output being common with another supply node of the primary windings.
In a further aspect, the present invention provides a traffic light system to provide traffic light signals, the traffic light system comprising:
at least one optical assembly comprising:
a plurality of lighting elements; and
an input transformer having primary windings and secondary windings, the secondary windings being electrically coupled to the plurality of lighting elements; and
at least one impedance matching circuit electrically coupled to the optical assembly, each of the impedance matching circuit comprising:
at least one matching transformer;
a first resistor chain comprising at least one resistor connected in series and having an input and an output, the input being common with an electrical node of the matching transformer, the output being common with a supply node of the primary windings; and
a second resistor chain comprising at least one resistor connected in series and having an input and an output, the input being common with another electrical node of the matching transformer, the output being common with another supply node of the primary windings.